Ion source



April 19, 1955 w. c. WILEY ION SOURCE Filed 001;; 1, 1951 WSR IN V ENTOR. a W/LE/ WALL/AM United States Patent ION SOURCE William C. Wiley,Detroit, Mich., assignor to Bendix Aviation Corporation, Detroit, Mich.,a corporation of Delaware Application October 1, 1951, Serial No.249,080

9 Claims. (Cl. 250--41.9)

This invention relates to apparatus for, and methods of, producingpulses of ions and more particularly to apparatus for, and methods of,producingrela tively sharp and strong pulses of ions. The invention isespecially adapted for use in conjunction with a mass spectrometer toproduce pulses of ions from the gases in an unknown mixture.

Mass spectrometers in general are used to determine the weights ofdifferent gases constituting an unknown mixture and to determine therelative abundance of the different gases in the mixture. Some massspectrometers now in use operate on the principle of forming a pulse ofions and subjecting the ions to a predetermined force. The ions ofrelatively light mass are given a greater acceleration by thepredetermined force than the ions of heavy mass and are collected beforethe ions of heavy mass after travelling through a predetermineddistance. Thus, by measuring the relative times at which the ions arecollected, the masses can be determined.

In order for accurate determinations to be made, the pulses of ionsutilized must be formed from a relatively large number of ions and mustbe confined within a relatively narrow space. Until recently, apparatusdid not exist for producing such sharp and strong pulses. However, inco-pending application, Serial No. 221,554, filed April 18, 1951, by meand Ian H. McLaren, an ion source is disclosed in which a relativelylarge number of ions is stored in a relatively small space. In the ionsource disclosed in the above application, the ions are retained in thepotential well created by a stream of electrons and are withdrawn inpulse form after the electron stream is cut off.

One disadvantage limiting the successful operation of the ion sourcedisclosed in the above co-pending application is the tendency of theions to gradually move towards the grids which accelerate the electrons.As a result of this movement, the number of ions available forwithdrawal in pulse form decreases and produces 9. correspondingreduction in the strength and accuracy of the measurements obtained.This invention overcomes the above dis advantage by providing an ionsource which confines the ions to a relatively destricted region alongthe length of the electron stream. I

An object of this invention is to provide an ion source for producingpulses of ions having a relatively narrow width.

Another object is to provide an ion source of the above character forproducing pulses of ions formed from a relatively large number of ions.

A further object is to provide an ion source of the above character forretaining the ions in a relatively confined space after they areproduced so as to obtain strong and accurate measurements from the ionpulses.

Still another object is to provide a method of concentrating a maximumnumber of ions in a minimum space to produce a strong and sharplydefined pulse.

Other objects and advantages will be apparent from a detaileddescription of the invention and from the appended drawings and claims.

The single figure is a view, partly in perspective and partly in blockform, illustrating somewhat schematically an ion source constituting oneembodiment of this invention and a mass spectrometer in which it may beused.

In the embodiment of the invention illustrated in the to the cathode 10and is provided with a vertical slot 14 whose median position isdisposed at substantially the same horizontal level as the cathode 10. Acontrol grid 16 is positioned relatively close to the accelerating grid12 on one side and relatively close to a shield grid 18 on the otherside and is substantially in alignment with both grids. The grids 16 and18 have slots 20 and 22 corresponding substantially in shape andposition to the slot 14. A collector plate 24 substantially in alignmentwith the grids is provided at a relatively great distance from the grid18 to receive the electrons travelling past the grid.

A backing plate 26 is disposed in parallel with, and slightly to therear of, the collector plate 24, and an ion accelerating grid 28 havinga horizontal slot 30 is positioned slightly in front of the electronstream and substantially in alignment with the plate 26. A collectorplate 32 is provided substantially in alignment with the grid 28 at arelatively great distance from the grid. A time indicator 34, such as anoscilloscope, is connected to the collector plate 32 to indicate therelative times at which ions of different mass reach the plate.

The plate 26 and grid 28 form a compartment with the grid 18 and thecollector plate 24, the upper and lower limits of the compartment beingdefined by horizontal insulating plates 36. The lower plate 36 has ahorizontal slot 40 which communicates with the flared mouth 42 of aconduit 44. The conduit in turn extends from a receptacle 46 adapted tohold molecules of the different gases constituting an unknown mixture.

In the steady state condition, positive voltages of substantially equalmagnitude are applied to the grids 12 and 16 from a suitable directpower supply 48. In addition, slightly positive voltages are applied tothe collector plates 24 and 32. The cathode 10, the grid 18, the backingplate 26 and the ion accelerating grid 28 are all substantially atground potential in the steady state condition.

During the steady state operation of the mass spectrometer, electronsemitted by the cathode 10 are-accelerated towards the control grid 12because of the positive voltage on the grid relative to the cathode. Theelectrons are not further accelerated after they have travelled past thegrid 12, since the grid 16 is at approximately the same voltage as thegrid 12 and the grid 18 is substantially at ground. Therefore, anyelectrons which do reach the region between the collector plate 26 andthe grid 28 do not have a suificient force to produce ionization of amaterial number of gas molecules.

To produce a heavy flow of electrons towards the collector plate 24,voltage pulses of approximately equal magnitude and of negative polarityare applied on the cathode 10 and the grid 12 from a pulse formingcircuit 50. Upon the application of the negative pulse on the grid 12,the grid 16 becomes positive with respect to the grid 12 andconsiderably accelerates the electrons as they travel towards it.Because of this additional acceleration, the electrons travel into theregion between the plate 26 and grid 28 with sufficient energy to ionizemany of the gas molecules introduced into the region from the receptacle46. The electrons flowing from the cathode 10 and the electrons producedby the ionization of gas molecules are collected by the plate 24.

Because of their positive charge, the ions produced by the action of theelectron stream on the gas molecules are retained within the stream. Theions are retained in a space having a relatively narrow width in thedirection perpendicular to the electron stream because of thecollimating action which is provided on the stream by the slots 14, 20and 22. Furthermore, a relatively large number of ions can be, and is,retained within the electron stream, or negative potential well, beforethe negative charge of the stream becomes neutralized. This causes theion pulse withdrawn from the stream to have a strength considerablygreater than ion pulses formed by previous sources.

The ion pulses which are formed by the ion source disclosed above have arelatively great strength for another reason. Because of the positivevoltage on the collector plate 24 and the grid 16 during the electronstream, the ions are retained within the stream in the region adjacentthe slot 30. Since the ions cannot drift down the electron streamtowards the grids 18, 16 and 12, practically all of the ions formed fromgas molecules are included in the ion pulse which is withdrawn towardsthe collector plate 32. The action of the positive voltage on the grid16 serves to materially increase the number of ions available forwithdrawal in pulse form towards the collector plate 32.

When the electron stream has become substantially saturated by positiveions, the voltage pulses on the cathode and grid 12 are cut off and theelectron stream is discontinued. A negative voltage pulse may also beapplied to the grid 16 upon the discontinuance of the voltages pulses onthe cathode 10 and grid 12 to insure that the electron stream remainscut ofi even with the attractive forces produced by the positive voltagepulses which may be applied to the plate 26 and the grid 28. Since theions are no longer retarded by the negative voltage produced by theelectron stream, they are easily accelerated towards the grid 28. Thisacceleration is produced by the application of an electric field betweenthe plate 26 and the grid 23 at substantially the instant that theelectron stream is cut off or at a slightly later time. The electricfield is imposed on the ions by the application of a positive voltagepulse on the plate 26 relative to the voltage on the grid 28 or by theapplication of a negative pulse on the grid 28 relative to the voltageon the plate 26.

The electric field imposed upon the ions causes the ions of relativelylight mass to be given a greater acceleration than the ions of heavymass. As a result, the ions of relatively light mass reach the collectorplate 32 before the ions of heavy mass. By indicating the relative timesat which the ions of different mass reach the collector plate, themasses of the ions can be determined. The relative abundance of thedifferent gases in an unknown mixture can also be determined bymeasuring the amplitudes of the signals produced at the collector plate32 when the ions of different mass impinge on the plate.

It should be realized that the relationship between the voltages on thegrids 12, 16 and 18 during the application of the voltage pulses can bevaried from that disclosed above without materially affecting theperformance of the ion source. However, in any such variations the grid16 must have a positive voltage during the time that the ion pulse isbeing formed so as to confine the ions in the region adjacent the slot30. It should also be realized that other means than the plate 26 andthe grid 28 can be employed to withdraw in a pulse the ions that areformed. As previously disclosed, first pulses are initially applied tothe cathode 10 and the grid 12, and second pulses are subsequentlyapplied to the backing plate 26 and the grid 28. Equipment for producingtwo groups of pulses separated from each other by a relatively shorttime is known to persons skilled in the art and may be either purchasedor built. For example, Model 902 of the double pulse generatormanufactured by the Berkeley Scientific Company of Richmond, California,may be used to produce two groups of pulses having a variable timeseparation relative to each other. This model generator is fullydescribed in a publication entitled Instruction Manual, Berkeley DoublePulse Generator, Model 902 issued by the Berkeley Scientific Company inAugust, 1950. Equipment for producing a pair of puises having a variabletime separation may be built in accordance with the principles outlinedon pages 223 to 238, inclusive, of Volume entitled Electronic TimeMeasurements of the Radiation Laboratory Series prepared by theMassachusetts Institute of Technology.

Although this invention has been disclosed and illustrated withreference to particular applications, the principles involved aresusceptible of numerous other applications which will be apparent topersons skilled in the art. The invention is, therefore, to be limitedonly as indicated by the scope of the appended claims.

What is claimed is:

1. An ion source, including, means for forming a plurality of electrons,a collector plate having a positive voltage to attract the electrons,means operative upon the application of voltage pulses to produce achanneled acceleration of the electrons towards the collector plate, thelast mentioned means including a grid having a positive voltage toconfine any ions that are formed within a particular region between thegrid and the collector plate, means for introducing molecules of gas tothe electron channel for the production of ions and the retention of theions in the electron channel, and means operative at the end of thevoltage pulses on the electron accelerating means to withdraw the ionsin a pulse.

2. An ion source, including, means for producing a plurality ofelectrons, means, including a grid, for channeling the fiow ofelectrons, means for introducing a plurality of molecules into theelectron channel for ionization by the electrons and for retention ofthe ions in the electron channel, means for applying a positive voltageto the grid during the flow of electrons to accelerate the electronsthrough the channel and to confine the ions within a particular portionof the channel, means for cutting off the flow of electrons when thenumber of ions in the channel approaches saturation, and means operativeat a particular time relative to the cut-ofi of the electron flow toremove the ions in a bunch from the electron channel.

3. An ion source, including, a compartment, means for introducing aplurality of gas molecules into the compartment, means for forming aplurality of electrons, a collector plate located at one end of thecompartment and having a positive voltage applied to it, a grid locatedat the other end of the compartment and having a positive voltageapplied to it, the collector plate and grid operating in conjunction toretain in the center of the compartment any ions that are formed, means,including the grid, for producing a channeled flow of a pulse ofelectrons towards the collector plate with sufficient energy to produceions from the gas molecules, and means for withdrawing ions of the gasmolecules from the center of the compartment after the flow of theelectron pulse through the compartment.

4. An ion source, including, a cathode for emitting a plurality ofelectrons, a first grid spaced from the cathode, there being a hole inthe grid for the passage of the electrons from the cathode in a directedstream, a second grid spaced from the first grid, there being a hole inthe second grid substantially in alignment with the hole in the firstgrid, a third grid spaced from the second grid, there being a hole inthe third grid substantially in alignment with the hole in the secondgrid, a collector plate for collecting the electrons, means for applyingvoltage pulses between the cathode and the first grid and between pairsof successive grids to accelerate the electrons past the grids, thesecond grid having a positive voltage applied to it to repel any ionsthat are formed, means for introducing molecules of gas into the streamfor the production of ions and the retention of ions in the stream, andmeans fOli withdrawing the ions from the electron stream in a pu se.

5. An ion source, including, a cathode for emitting a plurality ofelectrons, at first grid spaced from the cathode, a second grid spacedfrom the first grid, a third grid spaced from the second grid, acollector plate spaced from the third grid, there being slots in thefirst, second and third grids to produce a beam of electrons from thecathode, means for applying voltage pulses at particular mstants betweenthe cathode and the first grid and between pairs of successive grids toproduce an acceleration of a pulse of electrons towards the collectorplate, means for introducing a plurality of molecules into the potentialwell created by the electron beam for ionization by the beam and forretention in the space occupied by the beam, means for applying apositive voltage on the second grid during the acceleration of theelectron beam towards the collector plate to prevent a drift of the ionsalong the beam towards the grids, and means for withdrawing the ions ina pulse at a particular time relative to the pulse of electrons.

6. An ion source, including, a cathode for emitting a plurality ofelectrons, a first grid disposed relative to the cathode to pass astream of electrons emitted by the oathode, a second grid disposedrelative to the first grid to pass the stream of electrons moving pastthe first grid, a third grid disposed relative to the second grid topass the stream of electrons moving past the second grid, means forcollecting the electrons moving past the third grid, means forintroducing a plurality of molecules into the electron stream forionization of the molecules and for retention of the ions within thestream, an electrical cir- (suit for imposing negative voltages ofrelatively short duration on the cathode and the first grid for themovement of the electrons to the collecting means, an electrical circuitfor imposing positive voltages of relatively moderate magnitude on thesecond grid and the collecting means to retain the ions in a particularregion along the length of the stream, and an electrical circuit forapplying an electrical field upon the ions in a direction transverse tothe electron stream to withdraw the ions in a pulse from their place ofretention.

7. An ion source, including, means for forming a plurality of electrons,electrical means for withdrawing the electrons in a stream, electricalmeans for collecting the electrons after their movement through aparticular distance, a backing plate disposed in substantially parallelre lationship to the electron flow, a first grid disposed insubstantially parallel relationship with the backing plate on the farside of the electron stream relative to the backing plate, means forintroducing a plurality of molecules into the region between the backingplate and the first grid for the ionization of the molecules and for theretention of the ions within the stream, a second grid disposed on thefar side of the backing plate and the first grid relative to thecollecting means, an electrical circuit for applying slightly positivevoltages to the collecting means and to the second grid to retain theions in a concentrated region in the electron stream, and an electricalcircuit for applying an electrical field between the backing plate andthe first grid to withdraw the ions in a pulse from their place ofretention.

8. An ion source, including, a cathode for emitting a plurality ofelectrons, a grid disposed at a particular distance from the cathode andshaped to channel into an electron stream the electrons emitted by thecathode, a col lector disposed at a particular distance from the gridand in substantially parallel relationship to the grid to receive theelectrons, a backing plate disposed between the grid and the collectorin substantially perpendicular relationship to these members and on oneside of the electron stream, a second grid disposed in substantiallyparallel relationship to the backing plate on the other side of theelectron stream, an electrical circuit for applying a voltage pulse onthe cathode relative to the voltage on the first grid to produce amovement of the electrons through the region between the backing plateand the second grid to the collector, means for introducing a pluralityof molecules into the region between the backing plate and the secondgrid for ionization of the molecules and for retention of the moleculeswithin the electron stream, an electrical circuit for imposing positivevoltages of mod erate magnitude on the first grid and the collector torestrain any movement of the ions towards these members, and anelectrical circuit for applying an electrical force between the backingplate and the first grid to produce a movement of the ions in a pulsefrom their place of retention.

9. An ion source, including, a cathode for emitting a plurality ofelectrons, a first grid disposed at a particular distance from thecathode and shaped to pass the electrons from the cathode in a stream, asecond grid disposed at a particular distance from the first grid andshaped to pass the electrons from the cathode in a stream, a third griddisposed at a particular distance from the second grid and shaped topass the electrons from the cathode in a stream, a collector disposed ata particular distance from the third grid to receive the electronsflowing past the third grid, a backing plate disposed between the thirdgrid and the collector in substantially perpendicular relationship tothese members and on one side of the electron stream, a fourth griddisposed at a particular distance from the backing plate on the otherside of the electron stream, an electrical circuit for producingelectrical fields between the cathode and the first grid and between thefirst and second grids to produce a movement of the electrons to thecollector, means for introducing a plurality of molecules into theelectron stream in the region between the backing plate and the fourthgrid for ionization of the molecules and for retention of the ionswithin the stream, an electrical circuit for positively biasing thesecond grid and the collector to prevent the movement of the ions alongthe stream towards these members, and an electrical circuit forproducing an electrical field between the backing plate and the fourthgrid to withdraw the ions in a pulse from their place of retention,

References Cited in the file of this patent UNITED STATES PATENTS2,535,032 Bennett Dec. 26, 1950 2,582,216 Koppius Jan. 15, 19522,642,535 Schroeder June 16, 1953 OTHER REFERENCES An Ion Velocition byCameron et al., published in The Review of Scientific Instruments, vol.19, No. 9, September 1948, pages 605 to 607.

